Reversal of rapid axonal transport at a lesion: leupeptin inhibits reversed protein transport, but does not inhibit reversed organelle transport

Abstract

The hypothesis that the reversal of rapid axonal transport requires a proteolytic conversion of anterograde transport vesicles was examined using sciatic nerve preparations from Xenopus laevis and leupeptin as an inhibitor of proteolysis. The transport of newly synthesized 35S-labeled proteins was studied with a position-sensitive detector of radiation. Organelle transport in isolated myelinated axons was studied by video microscopy. Leupeptin (0.1–0.4 mM) reduced the anterograde-to-retrograde reversal of protein transport adjacent to an axonal lesion. In experiments in which organelle transport was observed close to lesions in axons maintained in a medium compatible with intracellular function, 1.0 mM leupeptin inhibited neither the anterograde-to-retrograde nor the retrograde-to-anterograde reversal of organelle transport. In addition, in experiments in which conditions approximated those used to study protein transport, organelle transport away from the lesion was not inhibited by 1.0 mM leupeptin. A comparison of the morphology of rapidly transported organnelles that underwent anterograe transport to the morphology of those that returned from a lesion (with or without the presence of leupeptin) provided no evidence that a morphological conversion was a necessary step in transport reversal.